Something is off with those Uncle/Niece marriages though. We don't have that full story. First, they follow Cregan who lived a very long time and had multiple wives with multiple issue. This made the girls and their uncles only 'half' related. They share only Cregan as an ancestor. This may be why it was allowed. I may be off here, but I wonder if it was an Alys Karstark situation. Cregan died and these girls were the heirs to House Stark. The two half uncles married them to consolidate power in the House. It never happened again and seems an extraordinary circumstance with these two brothers and two sisters.
Cousins are common though. Tywin and Joanna were cousins. Had things been more out in the open Jon could have easily married one of the Stark girls.

Yeah, she's pretty but that doesn't make people love you. GRRM has that beauty and the beast thing going on. Ygritte isn't gorgeous, Jon is plain. Brienne is beautiful of soul and Jaime is beautiful of face. Cersei is gorgeous but rotten inside. Robert was a young god of a man and then got drunk and fat. Danaerys is the most beautiful woman in the world and the biggest monster inside. As Arya becomes more deadly she becomes prettier. Beware the pretty ones seems to be a message in George's work. I can totally see Sansa, proclaimed as gorgeous die alone.

I don't even think its a nice idea but I've been through it 10 times and I don't want to dredge it back up. Suffice to say, I don't like that Arya, the unconventional character is cast out of Westeros rather than forcing Westeros to change. Nymeria would roll in her grave.

I saw that coming in the books. Sansa dreams of love but no one has ever really loved her besides familiarly. Everyone is using her. She is really the lone wolf, surrounded by predators. This was a key reason I thought the sisters would invert their world views.

She doesn't need it though. That is a good contrast between her and Danaerys. Both began saying that they wanted the love of the people as Queen. In the end Sansa doesn't need love. One of her brother's died for it, one was destroyed by it, her sister seems to have broken someone with it. Daenerys on the other hand can't cope without being loved. It gives her validation. When she doesn't have it, we saw what happened. Summed up by this quite aptly

Elizabeth didn't die a virgin she just appeared so to her people. She married her country. In those days if she married then her husband would be King, a King outranks a Queen. She would lose her power. As for Sansa not getting what she wants, she did, 'I'd be queen someday, its the only thing I've ever wanted', she had to endure the sacrifices to get what she wanted. Some things, she will never have.

I can kind of understand Bran just shrugging but the Crown is Bankrupt. Littlefinger, Cersei and the wars have drained the treasury. Plus one of the few cities in Westeros has been destroyed. Bran is not rolling in it. The North similarly has lost pretty much its whole army and food supplies. In the books it cost Farman 3 priceless dragon eggs to get a crew and a ship. How would Arya know what ship to get? How would she know who to hire? The end of the show is her with maps, picking up a telescope. Arya does not know how to navigate at sea. And if it were the case she hired someone, well why isn't she with this person in the map room? Why is she at the prow as if she's in control. If that voyage goes badly, how exactly can she command the boat?
I wonder if the Farman stuff was George's version of the GOT ending done more realistically. This woman spent her whole life sailing, she sailed around Westeros and to the free cities. West of westeros was her dream. Arya had a passing thought and no skill to undertake the endeavour. I highly doubt he would give Arya a second hand ending.

This doesn't ring true to me because of how her arc with the Faceless men ended. 'I am Arya Stark of Winterfell and I am going home'. Her journey to becoming no one only reinforced who she was. Its why she couldn't let go of Needle. There never seemed to me to be any question of self discovery, her proclamation was so powerful. What was the point of it otherwise?
Yet the story ends with her completely rejecting that and disappearing. How did we get there? I have no clue.
The script for 6 confirms my suspicions that she's never intended to come back. I mean, she said that in episode 4. It feels to me like her whole arc was made worthless. She leaves home, her dad is killed and she embarks on a long journey to get back. She gets back, decides she doesn't actually care and pisses off god knows were. What was the journey for? Why have we watched this? Where is the resolution?

Kind of. I despise my favourite character's ending so I'm less excited about the journey to get there. Although, has to be said we still don't know what Arya is for in the books, there is no Night King. She doesn't seem to have a role after the war. So out of the core 5 she is still a massive question mark.

I think Sophie's comments are supported by the canon of the show though. Her coronation with her long freeflowing hair...
https://www.historytoday.com/sites/default/files/liz1_coronation.jpg
She never married and never had a child.
Book Wise, the last marriage out of House Stark to produce children was Jocelyn, the heirs in the Vale Cat mentions when Robb is writing his will.

What agenda? You've left a big post so I'll take it bit by bit.
I don't agree that most people went to America just because they could and left everyone they cared about behind. People don't just choose to leave their families without reason, be that crippling poverty or persecution. The show never provided Arya sufficient motivation to take that step. You suggest here she has urge to travel, we've never seen that before beyond a passing comment 3 seasons ago. I'd like to go to Rome one day, is that my destiny now mapped out forever? That is what you took from that scene. I've seen others argue that Arya is disconnected from humanity. The range of interpretations of that scene to me indicates its failure. We don't really know why she goes. She never tells us why she wants to get on a boat and never see Jon or Sansa again.
What is to explain is Arya's priorities. She's not booking Air BnBs online. This is a HUGE life changing choice where she will leave everything and everyone she knows behind. What for? I don't think 'I want to travel' is good enough. Especially for a character whose journey has been the antithesis of that. Arya has wandered around trying to get home, to get to her family again? Why? So she can do a head count and leave? This conclusion is a massive 180 from that and it needs some explanation. Just saying she's curious about the west is not good enough for such a massive shift. And to add to that, no one seems to worry or care about what she's doing. Nor do they try to stop her. That doesn't seem natural if one of my family says they are leaving to never return.
OK, catchphrases. When she has to tell Sansa how to use a knife. Why would Sansa need to know how to stab something? A contrived excuse to use 'Stick em with the pointy end'. It could have been a meaningful goodbye between the two sisters. No, catchphrase. Same episode 'Not Today'. To be fair I thought that one fit but its part of the trend. Thirdly, 'That's not me'. This is nonsensically brought up last season with Nymeria the wolf too. Why would Nymeria the wolf not go home to protect her family? Nymeria being a reflection of Arya. Again, seems to be their set up of Arya's conclusion. A lousy one considering Arya doesn't end up like Nymeria the wolf at all. But remember when that happened and people were wondering whether it meant it wasn't Nymeria at all? Well she reiterates it in 8x04 to similar fan debates as to its intended meaning. Why not just say something straight? Its not like anyone else knows that is what you said to your dad when you were pre-pubescent. It may need a bit more. The point is collectively these are all things Arya said when she was 11. I don't know about you but I don't want to be held to crap I said when I was 11. Why are the writers doing this? Can you imagine if Sansa was saying the same nonsense she said when she was 12? Sansa was allowed to grow. Arya remains stuck. The Hound has to tell us she has changed because she doesn't talk as much? To be honest I had not noticed a difference in that regard but I guess they assumed silence gave her some badass gravitas.
Your example of Frodo and Bilbo doesn't hold up if you think this is a positive ending for the character. Those characters were broken by their experiences. Going to the West was a purgatorial experience to get to heaven. Now Arya is an 18 year old woman in the show, she has been through a great deal and if they showed she was broken like Frodo then so be it. Tolkein explains why he leaves, that scar he got on Weathertop was a continuing reminder of his trauma that would never go away. Arya doesn't have that arc, its meant apparently to be seen as the opposite, Arya was meant to realise her life has worth beyond her vendettas. That she was to live again. A Frodo like ending doesn't work for her. She is saved from a Frodo like fate.
We do suspect the voyage is suicide because we know of 2 people who have attempted it and never returned. What else are we to think? Again, I'm not sure as to the agenda. I think her arc in season 8 was rather well mapped out until that curveball of a last scene. As I said before, I never really thought about her ending because the books don't seem to be ending. All I assumed is that she would be home. That she would be like Nymeria, having been cast out of home by her enemies she wanders the world, finds her place and family and never goes back. However we didn't see that. Arya's story didn't conclude. If we are to assume her story is still about finding home we either have to think she'll never find a place or her journey has only just begun making what we actually saw unsatisfying as a conclusion. Even Nymeria the wolf follows this trajectory, she's cast out of her home, wanders, finds a new one and a new family. Arya doesn't. She is cast out of her home, she wanders, she's still wandering with no indication there is an end point.
I also read a criticism a few weeks back that stuck with me about how Arya rejects patriarchy but instead of being like Nymeria and instigating change she is exiled. She doesn't fit in, won't play the system, doesn't want to change the system and so she needs to leave. Arya is probably the most overtly feminist character in the series, books and show, so that is jarring too. I hate this ending on a whole number of levels to be fair, I could be here all day. There is my agenda, to monopolise your time!

People usually feel forced to do that though. Talking about America, there were families on the Mayflower and religious idealists who felt like they had nowhere else to go but they knew there was something on the other side. It was a risk but they were an end point. Then there were the convicts who were forced over there. But even then, a boat to the US takes 2 months in the 17th century. If you want to come back, its risky to cross of course but the option is there, tradespeople did it over and over. A lot of the original English colonies were set up by tradespeople. I can't think of anyone today who willingly leaves there family never to return. Syrian refugees?
I think the problem here is the writing. It doesn't explain Arya's choice. She is reduced to catchphrases that don't mean much in season 8. She once said she was curious about the west, that is it. A passing comment to a stranger 3 seasons ago and we are meant to now think this is a life goal of hers? Not only that she has to throw away her previous primary motivation of home and family without explanation. Then there is the arc Arya goes on when she gets back from Braavos in becoming human again. The soldiers in season 7, Hot Pie, Jon, Gendry, Sandor, they reconnect her with who she is. So in her big revelation in 8x06 is to abandon civilisation and never see any of these people again? Not to mention the arc is about her choosing life after death and going on a voyage we suspect is suicide, the antithesis of choosing life. Its bizarre.

She'll die at sea. On top of her lines about not going back North there is the Robert and Lyanna parallel. She refused to marry the Lord of Storm's End and ran away to her doom. This is made explicit by these lines
Robert- I only know she was the one thing I ever wanted. Someone took her away from me, and seven kingdoms couldn't fill the hole she left behind.
Gendry- All I know is you're beautiful and I love you and none of it will be worth anything if you' re not with me
What they never made clear is why. Curiosity isn't enough to never see any of your family again.

Cersei didn't inherit anything. She crowned herself. A council should have been called but of course she didn't. It doesnt matter to Dany because a usurper is a usurper regardless. To her there is no difference between Cersei and Robert

Several things
1. She was never accepted as Queen by the realm. Heck Rhaenyra had a more significant reign and she isn't counted. She sacked Kings Landing, that doesn't automatically make her Queen. This is why coronations are done, to make a show of your legitimacy and have Lords publically swear allegiance
2. If we assume Dany was Queen and her heirs are legitimate its unclear how it would go. It most definitely wouldn't be Bran as he has no Targaryen blood. Jon outranks Dany and renounced his claim taking him out of succession, making her heir technically who she legitimised in Episode 4. Gendry, the great grandson of Rhaelle Targaryen and we are back in the Baratheon line.
3. There is no claim through marriage only by blood. Jon's claim comes from his father's blood. His mother's blood gives him a weak claim to the North as Eddard has 3 living children still. Were the situation normal the Starks could have married Jon to one of his cousins to create a Stark Lord. Bran can't just jump into the Targaryen line anymore than Dany can jump into the Stark line

Well half the kids of that generation were killed so only Ned and Benjen were left. When Benjen left to go the wall Robb had been born as well as Jon. There were three Starks in Winterfell at that time.
We also can't compare situations because they didn't grow up normally. They have been shunted, separated and abused. They all end up by themselves and not only that, Benjen visits Ned, It doesn't seem like Sansa will ever see Jon or Arya again. Maybe we'd feel less like this if Brienne and Pod were with Sansa at the end or Gendry was on the boat with Arya. Their isolation is uncharacteristic considering the flow of the story has been about them being unwillingly isolated from each other.

Cousins aren't considered incest in their world. Tywin and Joanna were cousins, no on cared. As you say, the Cregan Stark heirs seemed to be a case of consolidating claims and even then they weren't full uncles so there is some wiggle room, They are most definitely not in the Targaryen business.

North- Sansa or Rickon.
Iron Islands- Asha
Riverlands- Edmure
Stormlands- Edric
Dorne- House Yronwood
Reach- Someone through the Florent claim, maybe House Tarly. I thought it would have been Sam but it could be Dickon
Wessterlands- Tyrion I guess but it is a cop out on the whole Castamere thing if he lives. I know it would enrage Tywin but still. Incomplete foreshadowing
Vale- House Royce
I've always saw the First Men reclaiming the Kingdom because of the consistent allusion to them and memory and them having done The Long Night before.

Problem being, they have never shown us that the Starks would be happy to never see each other again. Where was that in the text? We are meant to believe that curiosity about the West or going to the Wall or being Queen matters more to these characters than each other? Where is that supported? We are left to suppose they really don't care about each other that much.
I think we all would have said that the trajectory of their stories was to get back home to each other.